1. Field of the Invention
The present invention relates to an improvement in a planetary gear apparatus including an internal gear having inner teeth which are constituted by pins or a combination of pins and rollers, and an external gear having trochoidal or arcuate outer teeth. More particularly, the present invention pertains to a planetary gear mechanism which is simplified at the output side thereof.
2. Description of the Related Art
A typical conventional planetary gear apparatus will first be described hereinunder with reference to FIGS. 3 and 4.
This planetary reduction gear is of a type in which a rotational force input to an input shaft 1 is transmitted to an output shaft 2 after its rotational speed has been reduced.
The mechanism for reducing the speed is as described below.
Eccentric members 3.sub.1 and 3.sub.2 are provided on the input shaft 1 at a predetermined phase difference (180.degree. in this embodiment). The eccentric members 3.sub.1 and 3.sub.2 are respectively in contact with external gears 5.sub.1 and 5.sub.2 through bearings 4. Each of the external gears 5.sub.1 and 5.sub.2 is provided with a plurality of inner roller bores 6, and an inner pin 7 and an inner roller 8 are fitted into each of the bores 6. The outer periphery of each of the external gears 5.sub.1 and 5.sub.2 is toothed in a trochoidal or arcuate form so as to form trochoidal or arcuate outer teeth 9. The outer teeth 9 engage with outer pins 11 provided on an internal gear 10.
The inner pins 7 are shrink fitted into a flange 12 of the output shaft 2.
In this known art, when the input shaft 1 rotates one revolution, the eccentric members 3.sub.1 and 3.sub.2 also rotate one revolution, but the external gears 5.sub.1 and 5.sub.2 are prevented from self rotating because of the engagement of the inner pins 7 with the inner roller bores 6. Therefore, in a case where the difference between the number of teeth of each of the external gears 5.sub.1 and 5.sub.2 and the number of outer pins 11 (acting as inner teeth) is one, while the input shaft 1 is rotating one revolution, the outer teeth 9 of each of the external gears 5.sub.1 and 5.sub.2 are displaced (are shifted) relative to the outer pins 11 of the internal gear 10 which constitute the inner teeth thereof by one tooth. In consequence, one revolution of the input shaft 1 is reduced to a factor which is the reciprocal of the number of teeth of each of the external gears 5.sub.1 and 5.sub.2, and the rotational force whose speed has been thus reduced is then transmitted to the output shaft 2 through the inner pins 7.
The planetary gear apparatus of the above-described type, however, retains the following disadvantages.
In a case where a power transmission mechanism constituted by the inner roller bores 6, the inner pins 7, and the inner rollers 8 is employed to transmit the rotational force, the flange 12 must be rigid enough to prevent the output shaft 2 from becoming deformed. For this reason, the size of members or portions near the output shaft 2 cannot be reduced in the diametrical direction of the shaft 2.
This power transmission mechanism requires highly accurate machining, and must also have a suitably high rigidity, since any slight deformation of a member or portion thereof has an adverse effect on the operation.
Further, the inner roller bores 6 occupy a relatively large space in each of the external gears 5.sub.1 and 5.sub.2, and hinder any reduction in the size of the external gears 5.sub.1 and 5.sub.2 and, hence, in the size of the overall apparatus.
FIG. 5 shows another known planetary gear mechanism (see the specification of copending U.S. Application Ser. No. 83901) filed Aug. 6, 1987, now U.S. Pat. No. 4,770,062. In this mechanism, a carrier 22 is mounted on an external gear 21. The carrier 22 is provided with spline grooves 22A, and the output shaft 23 is provided with spline grooves 23A. These spline grooves 22A and 23A are coupled by a drive 23 having splines 24A at opposite ends thereof, by which radial load is prevented from acting on the output shaft. However, the external gear 21 of this apparatus has a complicated shape, and it is therefore difficult to machine a large number of external gears 21 which are laid on top of another at one time. The external gear 21 and the carrier 22 must be machined in two separate processes. Since the spline grooves 22A are formed in the carrier 22, the pitch diameter of the drive 24 coupled to the spline carrier 22 is inevitably reduced. This requires that the external gear 21 and the drive 24 are subjected to carburization so as to enable the strength thereof to be increased, increasing the production cost.
FIG. 6 shows still another known planetary gear apparatus (see the specification of Japanese Patent Laid-Open No. 60-146939). This apparatus is arranged such that an arm-shaped carrier 26 is mounted on an external gear 25, and that the carrier 26 and an output shaft 27 are coupled by a drive 28. In this apparatus, machining of the external gear 25 is easy. However, a large space for the carrier 26 is required, and the size of the external gear 25 cannot be therefore reduced by a satisfactory degree in the diametrical direction thereof. Further, since the external gear 25 and the carrier 26 are two separate units, the number of parts is increased, making management of the parts difficult.